1. Field of the Invention
The present invention relates to a cathode ray tube which is preferably applied to an image pickup tube of electrostatic focusing/electrostatic deflection type for example.
2. Description of the Prior Art
The applicant of the present invention has previously proposed an image pickup tube of electrostatic focusing/electrostatic deflection type (S.S type) as shown in FIG. 1 (Japanese Pat. Appln. No. 156167/1983).
In FIG. 1, reference numeral 1 designates a glass bulb, numeral 2 a face plate, numeral 3 a target surface (photoelectric conversion surface), numeral 4 indium for cold sealing, numeral 5 a metal ring, and numeral 6 a signal taking electrode which passes through the face plate 2 and contacts with the target surface 3. A mesh electrode G.sub.6 is mounted on a mesh holder 7. Prescribed voltage is applied to the mesh electrode G.sub.6 through the metal ring 5, the indium 4 and the mesh holder 7.
Further in FIG. 1, symbols K, G.sub.1 and G.sub.2 designate a cathode to constitute an electron gun, a first grid electrode and a second grid electrode, respectively. Numeral 8 designates a bead glass to fix these electrodes. Symbol LA designates a beam restricting aperture.
Symbols G.sub.3, G.sub.4 and G.sub.5 designate third, fourth and fifth grid electrodes, respectively. These electrodes G.sub.3 -G.sub.5 are made in process where metal such as chromium or aluminium is evaporated or plated on inner surface of the glass bulb 1 and then prescribed patterns are formed by cutting using a laser, photoectching or the like. These electrodes G.sub.3, G.sub.4 and G.sub.5 constitute the focusing electrode system, and the electrode G.sub.4 serves also for deflection.
The electrode G.sub.5 is sealed with frit 9 at an end of the glass bulb 1 and is connected to a ceramic ring 11 with a conductive part 10 formed on its surface. The conductive part 10 is formed by sintering silver paste, for example. Prescribed voltage is applied to the electrode G.sub.6 through the ceramic ring 11.
The electrodes G.sub.3 and G.sub.4 are formed as clearly seen in a development of FIG. 2. To simplify the drawing, a part which is not coated with metal is shown by black line in FIG. 2. That is, the electrode G.sub.4 is made so-called arrow pattern where four electrode portions H.sub.+, H.sub.-, V.sub.+ and V.sub.-, each insulated and zigzaged, are arranged alternately. In this case, each electrode portion is formed to extend in angular range of 270.degree., for example. Leads (12H.sub.+), (12H.sub.-), (12V.sub.+) and (12V.sub.-) from the electrode portions H.sub.+, H.sub.-, V.sub.+ and V.sub.- are formed on the inner surface of the glass bulb 1 simultaneously to the formation of the electrodes G.sub.3 -G.sub.5 in similar manner. The leads (12H.sub.+)-(12V.sub.-) are isolated from and formed across the electrode G.sub.3 and in parallel to the envelope axis. Wide contact parts CT are formed at top end portions of the leads (12H.sub.+)-(12V.sub.-). In FIG. 2, symbol SL designates a slit which is provided so that the electrode G.sub.3 is not heated when the electrode G.sub.1 and G.sub.2 are heated from outside of the envelope for evacuation. Symbol MA designates a mark for angle in register with the face plate.
In FIG. 1, numeral 13 designates a contactor spring. One end of the contactor spring 13 is connected to a stem pin 14, and other end thereof is contacted with the contact part CT of above-mentioned leads (12H.sub.+)-(12V.sub.-). The spring 13 and the stem pin 14 are provided for each of the leads (12H.sub.+)-(12V.sub.-). The electrode portions H.sub.+ and H.sub.- to constitute the electrode G.sub.4 through the stem pins, the springs and leads (12H.sub.+), (12H.sub.-), (12V.sub.+) and (12V.sub.-) are supplied with horizontal deflection voltage varying in symmetry with respect to prescribed voltage. Also the electrode portions V.sub.+ and V.sub.- are supplied with vertical deflection voltage varying in symmetry with respect to prescribed voltage.
In FIG. 1, numeral 15 designates another contactor spring. One end of the contactor spring 15 is connected to a stem pin 16, and other end thereof is contacted with above-mentioned electrode G.sub.3. Prescribed voltage is applied to the electrode G.sub.3 through the stem pin 16 and the spring 15.
Referring to FIG. 3, equipotential surface of electrostatic lenses formed by the electrodes G.sub.3 -G.sub.6 is represented by broken line, and electron beam B.sub.m is focused by such formed electrostatic lenses. The landing error is corrected by the electrostatic lens formed between the electrodes G.sub.5 and G.sub.6. In FIG. 3, the potential represented by broken line is that excluding the deflection electric field E.
Deflection of the electron beam B.sub.m is effected by the deflection electric field E according to the electrode G.sub.4.
In FIG. 1, the ceramic ring 11 with the conductive part 10 formed on its surface is sealed with the frit 9 at one end of the glass bulb 1 in order to apply the prescribed voltage to the electrode G.sub.5. Since the machining process for the frit seal of the ceramic ring 11 is required, the manufacturing becomes difficult.
Further in FIG. 1, potential of the electrode G.sub.5 must be high and the potential difference between the electrodes G.sub.4 and G.sub.5 must be large in order to improve the focusing characteristics of the electron beam on the target surface 3. Since the collimation lens is formed between the electrode G.sub.5 and the mesh electrode G.sub.6 and the landing error of the electron beam is corrected, potential difference of some degree is required between the electrodes G.sub.5 and G.sub.6. Under consideration of above aspects, a cathode ray tube in the prior art is operated in such conditions that voltage E.sub.G3 of the electrode G.sub.3 =500 V, center voltage E.sub.G4 of the electrode G.sub.4 =0 V, voltage E.sub.G5 of the electrode G.sub.5 =500 V, voltage E.sub.G6 of the electrode G.sub.6 =1160 V, and voltage E.sub.TA of the target surface 3=50 V. Since the voltage E.sub.G6 of the mesh electrode G.sub.6 becomes considerably high in this constitution, discharge may be produced between the electrode G.sub.6 and the target 3 so as to flaw the target surface 3.